A porous honeycomb structure comprising: cell partition walls (ribs) forming a compound body of a plurality of cells adjacent to one another; and a honeycomb outer wall which surrounds and holds an outermost peripheral cell positioned in an outermost periphery of the cell compound body has been broadly used as a filter (diesel particulate filter (DPF)) for trapping/removing particulate matters contained in a dust-containing fluid such as a diesel engine exhaust gas, or a catalyst carrier for carrying a catalyst component which purifies toxic substances in an exhaust gas. As constituting materials, fire-resistant silicon carbide (SiC), cordierite and the like, or a compound material of these materials and the like are used.
Moreover, development of the DPF (DPF for regenerating a catalyst) has progressed adopting a regenerating system in which an oxide catalyst is carried by a conventional DPF, and deposited particulates are oxidized, burnt, and continuously regenerated.
As this honeycomb structure, for example, a porous silicon carbide catalyst carrier having a honeycomb structure has been described which is obtained, for example, by using a silicon carbide powder having a predetermined specific surface area and containing impurities as a starting raw material, forming this material into a desired shape, and drying and thereafter firing the material in a temperature range of 1600 to 2200° C. (Japanese Patent Application Laid-Open No. 6-182228).
Moreover, concerning the material constituting the honeycomb structure, for example, a cordierite compound material has been described which contains a cordierite matrix and a predetermined amount of sheet-like silicon carbide and which has a high strength at a high temperature and which is superior in creep characteristics (Japanese Patent No. 3065421).
In a sintering configuration (necking) by re-crystallization reaction of the silicon carbide powder itself in the catalyst carrier described in the Japanese Patent Application Laid-Open No. 6-182228, a silicon carbide component evaporates from the surfaces of silicon carbide particles, this component condenses in a contact portion (neck portion) between the particles, accordingly the neck portion grows, and a bonded state is obtained. However, since a very high firing temperature is required in order to evaporate silicon carbide, this raises costs. Moreover, since a material having a high coefficient of thermal expansion has to be fired at a high temperature, there has been a problem that a firing yield drops. When a high-porosity filter, especially a filter having a porosity of 50% or more is manufactured by the firing by the re-crystallization reaction of the above-described silicon carbide powder itself, the firing mechanism does not sufficiently function, therefore the growth of the neck portion is inhibited, and this has caused a problem that the strength of the filter drops. Furthermore, the above-described material is advantageous in that thermal conductivity is very high at 30 W/m·K or more and local heating is suppressed. However, when the material is used in the DPF for regenerating the catalyst, because of characteristics that a deposited amount of particulates is small and the material easily emits heat, a long time is required until temperature of the carrier rises, and a long time is required until the temperature is raised at a temperature at which the catalyst functions. Therefore, there has also been a problem that the particulates remain unburnt and regeneration efficiency drops.
It is to be noted that with regard to the DPF, it is one of important problems to reduce pressure losses largely influencing engine outputs as much as possible. To achieve the problem, it has been demanded that the DPF be set to a higher porosity, that is, a material having a higher porosity be used as a porous material constituting this filter. Concerning the DPF for regenerating the catalyst, it has been demanded that the pressure loss of the filter be suppressed as much as possible. It has also been demanded that a higher porosity, concretely a porosity of 50% or more, especially around 70% be set.
However, the porosity of the cordierite compound material described in the above-described Japanese Patent No. 3065421 is in a range of 0% (dense) to 50% (porous), and the material has not been satisfactory as the material constituting the DPF for regenerating the catalyst. Furthermore, the cordierite compound material described in the Japanese Patent No. 3065421 exerts a certain effect in enhancing creep characteristics or resistance to shock, but has a small content of silicon carbide, and has not been necessarily sufficiently satisfactory in respect of thermal conductivity or chemical durability.
As a method of constituting the honeycomb structure into the high porosity, there has heretofore been a method in which pore formers such as starch and foam resin are added to a raw material mixture of a porous material constituting this honeycomb structure and containing silicon carbide particles and the like, and these pore formers are burnt/flied at a firing time. However, to set the porosity to be not less than a certain degree, for example, 60% or more, an amount of pore formers to be added increases. When a large amount of organic compound based pore formers are added, amounts of organic volatile substances and gases such as carbon dioxide generated in a degreasing (calcining) stage also increase, and combustion heat also increases. Defective portions such as cracks, tears and cuts, that is, defective portions which do not exert a filter function and in which leakage of a fluid occurs are sometimes formed in a calcined (degreased) body or a fired body obtained on this preparation condition.
The present invention has been developed in consideration of the problems of the conventional techniques, and an object thereof is to provide a porous material which has a high porosity and a high strength and which has a remarkably low possibility of including defective portions such as cuts causing liquid leakage in a case where the material is used as a filter, and a method of manufacturing the porous material having the characteristics.